1. Field of the Invention
The present invention relates to observation optical systems, optical attachments and eyeball-characteristic detecting systems and, more particularly, to such systems which are suited to be used in the viewfinder optical systems of cameras or the like.
2. Description of Related Art
FIG. 1 is a longitudinal section view of the conventional single-lens reflex camera.
In FIG. 1, a camera body 101 with a photographic lens 102 contains a return mirror 103, a viewfinder system 104 composed of a focusing screen 105, a pentagonal prism 106 and an eyepiece lens 107, and a back cover 108. The photographer, while keeping his or her eye 109 in axial alignment with the eyepiece lens 107, observes an image formed on the focusing screen 105 by the photographic lens 102.
In such a holding position of the camera, as the photographer breathes through the mouth or nose, warm humid air ascends along the surface of the back cover 108, reaching the space between the eyepiece lens 107 and the eye 109 of the photographer. When the ambient temperature is low to make the temperature of the surface of the camera body 101 or the eyepiece lens 107 low, the cold surface of the eyepiece lens 107 partly cools the breath. So, in some events, dewdrops are deposited on the eyepiece lens 107 to make the eyepiece lens 107 fogged, so that it becomes difficult to observe the viewfinder image. Particularly, with a mask put on the nose and mouth for the purpose of protection against the cold, the eyepiece lens 107 tends to get fogged more easily. To avoid such a state from occurring, the photographer is obliged to stop breathing until a shot is completed. In another situation that, when the camera is moved from one place to another where the environment greatly changes in temperature or humidity, it also happens that condensation of moisture, not owing to the breath, takes place on the optical parts constituting the viewfinder.
To solve such problems, a method is known which uses solutions of surface active agents or the like having the antifogging effect. The solution is thinly applied to the surface of the eyepiece lens 107.
Besides this, as shown in FIG. 2, it can be also considered to use a hood 110 surrounding the periphery of the eyepiece lens 107 so that the breath hardly touches the eyepiece lens 107.
For the breath fogging problem that becomes serious when the camera is used at a low temperature, different methods are available as described above. However, any of these methods has the following drawbacks.
The method using the surface active agent does not insure that, as the outer surface of the eyepiece lens is somewhat deep from the frame thereof, the solution is uniformly applied to the entire area of that surface. The antifogging effect is ensured only temporarily. Further, the surface active agent when applied to the surface makes it a hydrophile. In other words, the wettability is enhanced, thus producing the antifogging effect. Therefore, a thin film of water is formed on the surface. As the condensed amount of moisture increases, the image to be observed possibly eventually distorts.
In the other method which uses the hood shown in FIG. 2, the length L of the hood 110 must be made longer than a certain value to obtain a sufficient antifogging effect. It varies depending on the conditions of the environment. For example, at a temperature of -10.degree. C., the desired length L is found to be 20 mm or longer. Otherwise, no antifogging effect can be obtained. In this case, the eye 109 of the observer is necessarily spaced away from the eyepiece lens 107 by a distance of about 25 mm. To measure how long the eye may be put away from the eyepiece lens to observe the entire area of the field of the viewfinder, a numerical variable called "eye relief" is usually adopted. For a viewfinder system whose eye relief exceeds 25 mm, its physical size becomes large. Therefore, a problem arises in that its price becomes high.
In the meantime, it has been known to provide the observation optical system such as the viewfinder of the camera with a visual line detecting device for detecting that location on the viewfinder image which the observer looks at, or for detecting the direction of a visual line of the observer.
With such a visual line detecting device, in detecting the current visual line of the observer, it becomes possible either to determine a specific focusing position on which the photographic lens is to be adjusted for focusing by the automatic focusing device built in the camera, or to selectively choose the menu displayed in the viewfinder. Thus, the photographer can operate the camera with higher mobility.
It is also known to provide a technique of detecting the refractive power of the eyeball of the observer, i.e., the diopter, to automatically drive a diopter correcting optical system, thus adjusting the diopter.
To make determination of the above-described eyeball characteristic such as the visual line and diopter of the observer, many principles have been proposed. Of these, there is a one that a light-emitting diode or like light source illuminates the eyeball of the observer and an optical system adjacent to the eye receives the reflected light from the eyeball and forms an image of the eyeball on a light receiving element (sensor). Using the image information as the signal from the sensor, the current eyeball characteristic is detected. This method is employed in many ordinary optical apparatus.
In the case of, for example, the camera, depending on the environment in which the camera works, it is possible that, as described before, fogging due to the temperature or humidity affects some optical parts of the above-described optical systems for illumination and for receiving the reflected light. If the fogging occurs, then not only the visual line and diopter become impossible to detect but also a faulty operation will take place to effect an unintentional result.